An early generation armor-penetrating 25 mm caliber class shoulder-fireable rifle is constructed from a longitudinal external housing, containing inside in slidable contact and coaxial basic components comprising a barrel, a bolt and a bolt carrier. Residing inside the bolt carrier are the bolt and the bolt bias springs that exert an axial urge on the bolt in a forward direction towards the barrel on the front side of the rifle. A rear anchored drive spring distal to the bolt carrier resides inside the longitudinal external housing and operates in axial contact with the bolt carrier.
Prior to firing, the bolt carrier has strips a cartridge from the magazine and chambered it in the barrel. The bolt and bolt carrier have both made axial contact distally with the barrel now stationary and biased forward in the battery position against a proximal hard limit stop on the proximal side of the rifle. When the cartridge is fired inside the rifle to launch a projectile, the static state of all slidable components in the rifle is disrupted, and the conservation of momentum results in a rearward oriented recoil momentum on the rifle that is equal and opposite to the momentum of the projectile and propellant traveling forward down the barrel towards the muzzle.
Axial barrel springs connect the barrel to the external housing, biasing the barrel towards the battery position at the front of the rifle, and giving it limited motion range towards the rear of the rifle. The barrel's rearward range is determined by a distal limit stop fixed to the external housing. Thus the rifle is at its maximum length when the barrel is at rest against the proximal hard stop prior to the firing of cartridge, and the rifle is at its minimum length when the barrel momentarily hits the distal hard limit stop and the barrel springs are maximally deformed by tension or compression. At this maximal barrel spring deformation, the barrel springs absorb their maximum in recoil energy by converting kinetic energy into potential energy.
The barrel's hitting of the distal hard limit stop separates it from the bolt, and bolt carrier distally connected to the drive spring. The bolt, bolt carrier and drive spring have been traveling rearward with the barrel as a unit in contact. The bolt, bolt carrier and drive spring continue their rearward travel, further compressing the drive spring, until a force balance is reached between the rear oriented recoil force from the moving recoil masses and the forward acting drive spring force.
This force balance point corresponds to the minimum length of the drive spring, which has absorbed its maximum in recoil energy by converting kinetic energy to potential energy through spring compression. The drive spring then converts its potential energy back to kinetic energy by extension from the minimum length, pushing the bolt and bolt carrier forward. In this forward travel the bolt carrier strips a new cartridge from the magazine and chambers it in the barrel, on its way to achieving contact between the bolt, bolt carrier and the distal end of the barrel and forming a firing or combustion chamber. This contact is accompanied by a substantial momentum transfer from the bolt and bolt carrier urged forward by the drive spring, imparting a forward momentum to the barrel.
Under normal operation of a semi-automatic rifle, target aiming for the next round takes place after this contact, thus precision aiming is not compromised. However this momentum transfer has not been recruited into any performance function or benefit. It is simply wasted. The drive spring remains in a compressed state through the entire operational sequence of cartridge loading, chambering, firing, recoil and barrel return to battery position.
The drive spring constant typically dominates the other spring constants in the rifle, as exemplified by the sizable rear compartment it resides, providing the drive spring with substantially larger cross-sectional area and length for a high spring constant and spring stiffness. The combined gravitational effect of the moving masses over the springs in the rifle do not present any malfunction nor significant operational parameter deviation from angle to angle such that the rifle functions properly over a wide range of angles of inclination meeting its performance specification and design goals.
The firing mechanism of early generation rifle comprises a firing pin without a bias spring residing axially inside the bolt with their lengths being rather similar, a pin-pivoting trigger with a sear notch, and a spring loaded pivotal sear with a sear notch capture around the middle that cooperates in a locked manner with the trigger's sear notch in the non-firing position. When the trigger is pulled for firing, the spring loaded sear is freed and slides, releasing the firing pin. The firing pin then plunges forward axially through the stationary bolt and bolt carrier, its tip impacting the cartridge primer and firing the projectile from the cartridge inside the rifle. The trigger and sear are located behind the magazine. After a new magazine is loaded into the rifle, a retracting handle attached to the bolt carrier enables the bolt carrier to be manually retracted back sufficiently to initiate loading a new cartridge for firing.
After the trigger is pulled and the firing pin rushes forward to fracture the primer, the charge is set off. The projectile is sent forward down the barrel, and the recoil sends the barrel, spent shell, firing pin, bolt and bolt carrier rearward. Prior to the firing, the longitudinal firing chamber is established by the distal drive spring force acting on the bolt carrier containing the bolt, and the frontal reaction force from the proximal hard limit stop acting on the barrel. As long as the firing chamber is closed it is non-essential to remain stationary relative to the rifle at the time of firing to achieve projectile range.
The early generation platform's basic structure and associated operational sequence described thus far form the basis upon which further modifications are made to meet changing performance and functions. Though relatively simple, this platform provides sufficient flexibility to accommodate a number of significant modifications without complete redesign of the basic structural platform, meeting more demanding performance requirements and functions, and has become the platform of choice to serve the military in the future.
In a modification to improve the locking capability and firing reliability of the firing mechanism, the pin-pivoting trigger comes in contact with a second pin, which is in contact with a normally parallel transfer bar having a spring loaded rear pivot mount. The transfer bar acts against a spring loaded sear mounted on the bolt carrier and is normal to the rifle's axis. The sear has a hook in cooperation with another hook at the distal end of the firing pin. An axially placed spring around the firing pin connects the firing pin to the bolt and provides a forward bias.
During firing operation the trigger is pulled, rotating the transfer bar through a mutually contacting pin, resulting in an upward motion of the transfer bar. This upward motion of the transfer bar pushes back the spring loaded sear, releasing its hook from the cooperating hook on the distal end of the firing pin. Once unlocked, the spring loaded firing pin follows the spring's forward bias to plunge its proximal tip into the chambered cartridge, fracturing the primer and firing the projectile. This modification reduces the uncertainty of the position of the firing pin at the time the trigger is pulled. The supporting mechanism provides the locking capability on the firing pin in a non-firing condition, further improving its positional stability under field shock and vibration, and the forward bias spring on the firing pin provides greater and more consistent impact force to achieve more reliable firing.
In a modification to implement reliable semi-automatic firing in the shoulder fireable, armor piercing rifle, the slidable bolt carrier and the firing assembly's stationary trigger and pivotal transfer bar are modified. A rearward spring biased pivotal locking lever is also added distally to the bolt carrier. The sear and firing pin are similar to previous spring loaded designs. The pin mounted pivotal trigger is added with a slender latch hook that essentially stands upright and rotates forward when the trigger is pulled. The rear mounted and spring loaded pivotal transfer bar is added with a proximal slender catch member to work with the new latch hook in the trigger. This slender catch member is inclined at a positive angle to the horizontal, with its distal end slanting down and cooperating with the latch hook from the trigger, its proximal end slanting up and cooperating with the bottom of a vertically oriented spring loaded sear for the firing pin. Both the trigger and transfer bar are fitted with lugs facing each other for upward rotation of the transfer bar. The bolt carrier on its bottom distal end is added with a pivotal cocking lever pointing downward at the trigger and potentially interfering with the pivotal transfer bar.
When the trigger is pulled to fire a cartridge, the top of the pivotal trigger rotates forward, inducing the top mounted latch hook to follow. With the slidable bolt and bolt carrier in a forward position chambering the cartridge for firing, the bolt carrier's rear mounted cocking lever is out of the way of the stationary pivotal transfer bar, allowing the transfer bar's translational bias spring to freely push it forward. In the transfer bar's forward position, the forward rotated latch hook of the trigger misses the catch member on the transfer bar, and stays beneath it. This allows the mutually facing lugs on the trigger and transfer bar to contact and to rotate the transfer bar upward. In turn the downward spring biased vertical sear is pushed upwards, releasing the spring loaded firing pin to plunge forward to fire the cartridge.
The recoil sequence has been studied for further improvement to achieve overall smoothness. As the barrel, bolt and bolt carrier are sent backwards during the recoil, the barrel's limited rearward travel is defined by a distal limit stop mounted to the longitudinal external housing. The harsh impact on this stop separates the barrel from the bolt and bolt carrier which continue their rearward travel.
When the cartridge is fired, the recoil sends the battery positioned barrel, bolt, bolt carrier and the self-unlocking rod rearward, with the moving members in contact with the barrel. The fired projectile typically leaves the muzzle of the rifle at about the first ½″ rearward travel by the barrel, making its trajectory immune to any shock events during the rest of the recoil. As the bolt carrier approaches the trigger, its rear mounted, down pointing self-unlocking lever crosses the space immediately in front of the interference shoulder. After an incremental travel, the self-unlocking lever engages the interference shoulder and is rotated forward and upward, with its frontal catch slot presented towards the self-unlocking rod.
Prior to the self-unlocking lever reaching its maximum forward rotation, its catch slot starts engaging the self-unlocking rod, which is still being pushed rearward by the barrel. Further forward rotation of the self-unlocking lever from the interference shoulder starts to gently separate the bolt carrier from the barrel. Momentarily later the bolt also separates from the barrel before the barrel hits the distal barrel limit stop. The limit stop is made of resilient rubber to further soften the impact.
At the point of barrel to bolt carrier separation prior to hitting the barrel limit stop due to the working of the self-unlocking rod, momentum is transferred from the barrel to the bolt carrier, decreasing the rearward velocity of the barrel resulting in a gentler impact on the barrel limit stop, and increasing the velocity in the bolt carrier. Upon the forward return stroke urged by the drive spring, the process is reversed, and the bolt and bolt carrier again strip a new cartridge from the magazine, and chambers it in the barrel, while the bolt and bolt carrier re-establish contact with the barrel and remain stationary in the battery position prior to the next firing.
In addition to smoothing the recoil profile, substantial investigations have been made to reduce the magnitude of the recoil. In yet another improvement, a variety of weapons have been developed over the years that use advanced primer ignition and functioning the rifle from open bolt, i.e. utilizing the forward momentum of the recoiling mass, to offset a portion of the recoil impulse from firing. One example is the 40 mm MK19 Grenade Machine Gun.
An example for illustration are the Barrett weapons currently manufactured for sale in the United States and other countries that weigh approximately 28 pounds, and function from the closed bolt position. When an efficient anti-recoil muzzle brake is used, the .50 caliber Model 82A1/XM107 produces recoil energy of approximately 35 foot-pounds. Recoil energy increases to approximately 60 foot-pounds if the muzzle brake is replaced with a sound suppressor. The 25 mm XM109 variant of this weapon produces recoil energy of approximately 80 foot-pounds with an anti-recoil muzzle brake. Without a muzzle brake, that is, with a bare muzzle, recoil energy increases to approximately 90 foot-pounds for the 30-pound version of the XM109.
The United States military generally will not permit soldiers to operate shoulder fired weapons with recoil energy in excess of 60 foot-pounds. Prior concepts for reducing recoil of the weapon involved increasing the recoiling mass, reducing the muzzle velocity of the projectile, improving the anti-recoiling effectiveness of the muzzle brake, and combinations of the preceding. Some of these approaches change the weapon's performance. Increasing recoil mass to reduce recoil velocity and recoil energy may cause earlier fatigue in the operator. By reducing the muzzle velocity of the projectile, the effective range is reduced. Depending on the application this may not be an acceptable solution. Higher anti-recoiling effectiveness of the muzzle brake may require altering the propellant charge characteristics to increase gas mass and gas pressure at the muzzle, together with shortening barrel length to maintain same muzzle velocity. Changing the charge introduces a variation to current variety, increasing the hurdle.
Hence there is still an unmet need to improve a smaller caliber weapon of less than 40 mm with high performance such as the Barrett Model 82A1 to further reduce recoil impulse and recoil energy while maintaining its advantages of a semi-automatic rifle that is shoulder fireable, armor penetrating or anti-armor, high targeting precision, relatively light-weight and small footprint. The need for such a weapon has heretofore remained unsatisfied.